It is generally known that soil pollution is a worldwide problem. The present way of dealing with this problem is that the soil is isolated, after which the worst pollution is removed and stored elsewhere. Although this method is inexpensive, it does not provide a genuine solution to the problem. Another, promising method is the in-situ conversion by means of biological techniques. Here a conversion of the chemical compounds takes place in the soil itself by means of microorganisms which are present in the soil or which are added thereto later. This method is highly effective especially in the case of organic pollutants.
The above in-situ biological conversion method, however, leads to a number of problems. The main problem is that the growth of microorganisms already present in the soil or introduced into it at a later stage does not or substantially not take place, so that their effectivity is similarly poor.
The growth and activity of these microorganisms can be intensified by the presence of electron donors or electron acceptors. The problem of a bad growth of microorganisms would thus seem to be susceptible of a ready solution but for the fact that the dosage and even distribution of said electron donors and acceptors is found to be problematic in practice.
An example of a conversion of chemical compounds by means of microorganisms where a certain amount of electron donors is to be added to the soil under treatment is the biological breakdown of chlorinated hydrocarbons. An anaerobic phase is necessary in the biological breakdown of chlorinated hydrocarbons. During this phase under exclusion of oxygen, large quantities of electron donors are to be introduced into the soil. Usually, an electron donor such as, for example, hydrogen or an organic compound such as acetate, ethanol, or the like, dissolved in water, is pumped into the soil. Often, however, a satisfactory dosage of the electron donor throughout the soil portion is not obtained in this way. Instead, the above pumping process often leads to a choking effect and accordingly to a stagnation in the cleaning process of the polluted soil.
The anaerobic phase in the biological breakdown of chlorinated hydrocarbons must often be followed by an aerobic phase for ensuring a full decomposition of the polluting compounds. This oxygen-rich phase is usually effected through the injection of compressed air. A disadvantage of this method of supplying oxygen is that chlorinated hydrocarbons are volatilized thereby, which requires counteracting measures involving additional expense.
In addition, a separation between the anaerobic and aerobic zones is difficult to realize, as is the control of the ground water flow through the two zones, so that there is a risk of incomplete dissociation and the spread of toxic intermediate products.